Wristwatch paging receivers are known in the art, as illustrated by U.S. Pat. No. 4,713,808 to Gaskill et al. Such receivers often use antennas that are disposed in a band looped around the user's wrist or neck. The impedance of such an antenna is strongly dependent on many factors, including the band's length, its degree of parasitic coupling to the user's body and other objects, and the frequency at which it is operated.
Wristband antennas are relatively small in terms of wavelength and may have a relatively high Q. Even a slight change in any of the above-referenced factors can result in a large change in tuned frequency. Furthermore, their small size provides a correspondingly small signal to the attached receiver. It is important that a tuning/impedance matching circuit be provided to match and tune the antenna to the receiver to insure optimum power transfer therebetween.
WIPO patent publication 88/05213 discloses a wristwatch receiver embodying a variable matching circuit. In the disclosed system, the matching circuit includes a variable capacitor that can be accessed by opening a cover element. Once exposed, the circuit can be tuned with a screwdriver. The WIPO disclosure proposes adjusting the capacitor by using an auxilary RF signal generator located near the receiver to radiate a signal at the desired reception frequency. The receiver displays the relative strength of the received signal on an LCD display positioned beneath the removable cover. With the RF signal generator activated, the user can adjust the variable matching capacitor until the display indicates that a maximum signal is being received.
The tuning procedure for the system disclosed in this WIPO publication requires manual intervention. It requires that a cover be removed from the watch and a microminiature capacitor be adjusted with a screwdriver. The receiver may become detuned as soon as it is moved, whether off the user's wrist or simply towards or away from the user's body. Each such movement changes the antenna's parasitic coupling to the surrounding materials and thereby changes its resonant frequency which requires that the tuning process be repeated.
A related WIPO publication, 88/05214, discloses a second wristwatch receiver embodying a variable tuning circuit. In this second system, an automatic tuning circuit within the wristwatch receiver is activated in response to reception of a tuning mode signal on the frequency being received. This tuning mode signal can be part of the system synchronization signal, an independent selective call signal decodable by an entire group of receivers, or a secondary signal decoded after a receiver's individual selective call address has been decoded. The tuning operation can also be manually initiated by a user operable switch.
Tuning is effected by a varactor tuning element that is controllably biased by a microcomputer through a D/A converter. The bias voltage is increased, apparently in uniform steps, until a drop is noted in the level of the received signal. The bias voltage that was applied immediately preceding this drop is then selected as the proper tune voltage.
The magnitude of the received signal in this second system is determined by comparing it against four discrete analog levels. If, in a given situation, all of the levels are exceeded, the apparatus must reduce the gain of the receiver until the received signal falls between the minimum and maximum comparison levels. Similarly, if the received signal is below all four levels, the apparatus must increase the gain of the receiver. This arrangement slows the tuning operation and renders the tuning system inoperative in areas of very high or low signal strength, where the receiver cannot bring the received signal within the range of the comparison levels.
Another characteristic of this system is its presumption that the received signal strength will only have one local maximum as the varactor bias voltage is increased. Consequently, as soon as a drop in received signal is noted, the system concludes that the optimum tuning condition has been determined. More likely, the received signal will have several local maxima, many of them spurious due to unpredictable effects such as noise and movement of the antenna during the tuning operation.
This second system requires that a tuning command signal be transmitted with the paging signals. The disclosure specifies that an unmodulated carrier should be transmitted for approximately 200 milliseconds following the command signal to permit the tuning apparatus to tune. This requires a redesigning of the paging protocol and uses time that could otherwise be used for message transmission. The more often the tuning command signal is transmitted, the more time is diverted from message transmission.
If a receiver according to this second WIPO disclosure is moved between tuning intervals, it may be operated in an untuned state until another tune command is received. A receiver operated in an untuned state may miss a message.
It is an object of the present invention to provide an improved automatic tuning apparatus for a radio receiver.
It is a more particular object of the present invention to tune a matching network in a wristwatch receiver periodically, such as at the beginning of every reception slot for which the receiver is energized.
It is another more particular object of the present invention to tune a matching network in a wristwatch receiver over its entire range before concluding which tuning condition is optimum.
It is still another more particular object of the present invention to increment the bias voltage applied to a varactor tuning element in steps that correspond to unit increments of capacitance, rather than unit increments of voltage.
It is yet another more particular object of the present invention to phase-lock to a clock signal broadcast with paging data simultaneously with the automatic tuning of an antenna matching network.